156 Mr. Mallet on the Physical Conditions 



It has (as compared with zinc aggregated in large lamellar crystals) a 

 slower solubility in acids. 



It has greater malleability, and a greater extreme range of temperature 

 within which it remains malleable. (Note H.) 

 Eut if the same zinc be not merely fused, but heated up to a red heat, and 

 so poured into the mould, then, when solid — 



Its crystalline grain on fracture is coarse, large, and lamellar. 



Its specific gravity is only 686. 



It dissolves more rapidly in acids than the former; and 



It has scarcely any malleability at any temperature. 

 And these results are the same, relatively, whether in either case the zinc mass 

 be let to cool slowly in the mould, or be taken out as soon as solidified, and sud- 

 denly cooled in water. 



M. Bolley thinks it probable that zinc may be dimorphous, taking the form 

 of the regular system when crystallizing from a high temperature, like copper, 

 gold, lead, silver ; and the rhombohedric, like bismuth, antimony, arsenic, tel- 

 lurium, when crystallized from just its fusing point, and so indicating relations 

 of a crystallometric character with platina, iridium, and palladium, whose atomic 

 volumes are almost the same as that of zinc. Whether this explanation, which 

 does not commend itself to me as probable, be so or not, the fact is clear, and, 

 coupled with our previous knowledge, may with confidence be applied to cast- 

 iron ; and the conclusion and rule be thence deduced, one of the utmost impor- 

 tance to obtaining serviceable cast-iron guns. 



30. That the lower the temperature at which the fluid cast-iron is poured into 

 the mould, and the more rapidly the mass can be cooled down to solidifica- 

 tion, the closer will be the grain of the metal ; the smaller its crystals, the 

 fewer and least injurious the "planes of weakness," and the gi'eater the specific 

 gravity of the casting, cceteris paribus. 



31. Practical iron-founders are in the habit of judging of what they deem by 

 experience the best temperature of the fluid iron for being poui'ed into the 

 mould, by a certain peculiarity in the form of the vorticose movements that go 

 on upon the surface of a mass of fluid iron, and called technically "the break- 

 ing" of the iron. This test, however, is perfectly empirical and fallacious. The 

 very lowest temperature at which the iron remains liquid enough fully to fill 



